With the progress of computer image processing techniques, there has recently been developed a method in which images can be directly written on a photosensitive layer by light irradiation corresponding to digital signals. Thus, an intense interest has been shown toward a computer-to-plate (CTP) system in which images are directly formed on a photosensitive lithographic printing plate precursor without outputting onto a silver salt mask film, by employing the method. The CTP system, which uses a high-output laser having a maximum intensity within a near infrared or infrared range as a light source for light irradiation, has the following advantages: images having high resolution can be obtained by exposure within a short time and the photosensitive lithographic printing plate used in the system can be handled in daylight. Regarding solid and semiconductor lasers capable of emitting infrared ray having a wavelength of 760 to 1,200 nm, a high-output and portable laser is available with ease.
By the way, as a lithographic printing plate precursor which can form images using solid laser or semiconductor laser, there has been proposed a lithographic printing plate precursor comprising a substrate, an image recording layer formed on the substrate, and a protective layer of the image recording layer so as to prevent scratching of the surface of the lithographic printing plate precursor.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2004-157459 discloses a lithographic printing plate precursor comprising a lower layer comprising a water-insoluble and alkali-soluble polyurethane resin and an upper layer comprising a m,p-cresol novolac resin, and this lithographic printing plate precursor is excellent in print run length and press life. However, this lithographic printing plate precursor has a problem in that it has narrow development latitude in a developing solution having the pH of 12 or lower, and also the upper layer is peeled off during the development to form deposits on a developing tank, and thus there is room for improvement in its developing properties.
In order to solve these problems, the inventors of the present application have proposed, in the specification of Japanese Unexamined Patent Publication (Kokai) No. 2007-17913 description, a lithographic printing plate precursor comprising a substrate, a first image recording layer formed on the substrate, and a second image recording layer formed on the first image recording layer, wherein the first image recording layer a resin which is water-insoluble and alkali-soluble or alkali-dispersible in an aqueous alkali solution, and the second image recording layer comprises a polyurethane which includes a unit comprising a substituent having an acidic hydrogen atom.
The lithographic printing plate precursor proposed in the specification of Japanese Unexamined Patent Publication (Kokai) No. 2007-17913 exhibits satisfactory developability using a developing solution having the pH of 11 or lower, and also has long print run length and satisfactory scratch resistance. However, an improvement from the viewpoint of ink receptivity is required.